When I see a car fast approaching in the rear-view mirror, I find I can’t help but make assumptions about the personality of the driver based on the model of car they’re driving. Now a new study suggests these kinds of stereotypes can affect our memory for how fast a car was travelling – a finding that could have important implications for the trustworthiness of eye witness statements.

In an initial experiment, Graham Davies played ten-second video clips of a BMW and a (smaller, less powerful) Volkswagen Polo to 42 undergrads and asked them to estimate how fast the cars were going. Based on past research showing that participants expect BMWs to be driven faster than Volkswagen Polos, Davies thought that the students would overestimate the speed of the BMW. In fact, he found the opposite. Participants tended to overestimate the speed of the Polo, perhaps because it was a noisier car, and smaller vehicles are generally perceived as going faster than larger cars.

A second experiment pulled out all the stops in an attempt to provoke participants to rely on their driver stereotypes. Participants were told that the BMW was driven by a young male, and the Polo by a 62-year-old; they were shown photos of the drivers; and they were asked to speculate about the drivers’ personalities. But even after all this, the participants’ judgements of the cars’ speeds were still accurate and there was no tendency to overestimate the speed of the BMW. This was true even though participants had earlier made the kind of assumptions about the two drivers that you might expect – for example, that the BMW driver was more aggressive and reckless.

The key finding emerged in the third experiment. This was similar to the first two, but this time participants were asked, unexpectedly, to estimate the speed of the cars a day after seeing the video clips. In this case, the BMW’s speed was estimated to be significantly faster (56 mph) than the Polo’s (50 mph), even though both cars were actually travelling at the same speed (60 mph). Davies was surprised that both estimates were below the cars’ actual speeds, but nonetheless the retrospective judgements appeared to have been influenced by the stereotypes held by participants’ about the cars and their drivers. Moreover, the real-world relevance of this finding is clear, given that it is this kind of retrospective judgement that eye witnesses are asked to make following a crime._________________________________